The History of The Atomic Theory

Democritus

Democritus, a philosopher that lived around 500 B.C. came up with a theory that all matter consisted of extremely small particles that could not be divided.
"atom" comes from the greek word atomos, which means "uncut", or "invisible." He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? He also used other theories and had little help.

Jan 1, 600

Aristotle

Aristotle, a philosopher that lived in B.C. times a little bit later after Democritus. Aristotle thought there was no limit of dividing matter. Many centeries of people accepted his idea.
Aristotle used a model that said also all substances were built up from only four elements, earth, air, fire, and water.

Jan 1, 1704

Issac Newton

Couloumb

In the 1780's Couloumb formulated "Couloumb's Law" which stated, " The force between two electrical charges is proportional to the product of the charges and is inversly proportional to the square of the distance between them."

Jan 1, 1803

John Dalton

John Dalton, a teacher born in England 1766, did simple experiments outside of school. He concluded that gas consists of individual particles, and that the ratio of masses of an elements in a compound were a fixed composition. Dalton's Theory became widley accepted and wrell- known, but later in time scientists had to revise and change a few of his ideas.

Jan 1, 1889

Ernest Marsden

Ernest Marsden worked with Ernest Rutherford on the structure of the atom, and was the first secretary of the Department of Scientific and Industrial Research. Marsden did this by observing that a tiny fraction of alpha particles fired at a thin gold foil were deflected straight back.

Jan 1, 1894

G.J Stoney

G.J Stoney first proposed that electricity was made of discrete negative particles he called electrons.

Jan 1, 1897

J.J Thomson

J.J thomson used an electric current to learn about atoms.Thomson concluded that the particles in the beam of his experiment had a negative charge. Thomson's hypothesis that particles came from atoms provided the first evidence that atoms were made of even smaller particles.

Jan 1, 1904

Hantaro Nagaoka

Hantaro suggests that an atom has a central nucleus, and that electrons move in obits around it. Hantaro did not conclude and make a real effort to spread his hypothesis, he used yearlier work and gave information to later scientists

Oct 30, 1904

Abegg

Discovered that inert gases had a stable electron configuration which lead to their chemical inactivity.

Jan 1, 1905

Albert Einstein

postulated that light was made up of different particles that, in addition to wavelike behavior, demonstrate certain properties unique to particles. He also brought forth the theory of relativity.

Jan 1, 1906

Hans Geiger

Developed an electrical device to "click" when hit with alpha particles.

Jan 1, 1911

Ernest Rutherford

In 1899 Rutherford discovers that uranium emits fast moving particles with a positive charge. Ruthetrford later hypothosized that the mass and charge at any location would be too small to change the path of the particles. He then predicted that most particles would travel in a straight path from thier source to a screen that lit up when struck, (Gold Foil Experiment).All in end Rutherford concluded that the positive charge of an atom is not spread throughout an atom, it is all in the nucleus.

Jan 1, 1913

Niels Bohr

Niels Bohr, a Danish physicist agreed with Rutherford's ideas except his was more consentrated on the electrons. In Bohr's model, electrons move with constant speed in a fixed orbit around the nucleus.( Each electron has a fixed energy level.) (Bohr did use and compare a lot of Rutherford's expiriments though.)

Jan 1, 1914

H.G.J Moseley

Using x-ray tubes, determined the charges on the nuclei of most atoms. He wrote"The atomic number of an element =the number of protons in the nucleus". This work was used to reorganize the periodic table based upon atomic number instead of atomic mass.

Jan 1, 1924

Louis De Broglie

Louis de Broglie, a frenchman proposes the idea that electrons have a property almost like a wave. ( using mathematical expressions.)

Jan 1, 1926

Erwin Schrodinger

Erwin, an australian-irish scientist develops and uses mathematical equations to describe and explain the motion of electrons in atoms, along with ideas from Louis de Broglie, his work leads to a model including the electron cloud model. The electron cloud is a visual model of the locations of electrons in an atom. The probabiltiy of finding an electron is higher in the denser, or more packed regions of the cloud of the model.

Jan 1, 1926

Ernest Marsden

Ernest, a British scentist contribution to Rutherford atomic model while expirementing with alpha particles,

Jan 1, 1932

James Chadwick

James Chadwick, a British physicist confirms Erwin's last model and the exitence of neutrons. He confirmed and agreed with him with not only thought but through many experiments and checking his own work.

Jan 1, 1938

Otto Hahn

discovered nuclear fission, in which the nucleus of an atom breaks up into two separate nuclei, while experimenting with uranium.